1. Field of the Invention
The present invention relates to an improved process of applying metallic coatings to metal articles and more particularly to an improved procedure in the hot dip metallizing process of steel and other metal articles and to hot dip metallized articles obtained thereby.
2. Description of the Prior Art
In copending application Ser. No. 337,172 there is disclosed and claimed a hot dip metallizing process which consists in principe in
A. passing the article to be metallized through a bath of a molten heavy metal, preferably lead, said metal having a specific gravity higher than that of the coating metal and being substantially non-corrosive to the metal of the article to be coated, and PA1 B. conducting the article from said molten heavy metal bath through a confined layer of the molten coating metal such as zinc, for instance, in a stack, the surface area of said confined coating metal layer being a small fraction of the surface area of the heavy metal bath, said confined layer floating on said heavy metal and being of predetermined and limited height, thereby limiting the time of contact between the metal article to be coated and the coating metal to a predetermined limited time to achieve a coating of a predetermined character and avoiding any attack of the equipment by the coating metal.
This process has the advantage that the contact between zinc and the steel of the strip is of extremely short duration. Said process also provides means for regulating the level of the zinc in the stack by compensating for the consumption of the available amount of molten zinc in the stack, as the galvanizing operation proceeds. Said process also provides for periodic and automatic addition of molten zinc to the stack, creating an overflow of the zinc above the stack top. Thereby, any top dross formed, which consists mainly of oxides, is carried along towards the zinc melting oven, where the top dross is de-oxidized back to metallic zinc for further re-use.
It has been found that, under certain conditions, the strip to be metallized will pick up physically (or mechanically) small amounts of molten lead usually in the form of droplets which adhere to the metal strip to be metallized. Such adhering lead droplets, or the like, when penetrating into the zinc bath floating on the lead in the stack, will prevent uniform metallizing or galvanizing of the strip surface by forming "blisters" on the finished strip surface, a feature inadmissible for producing prime metallized articles.
Likewise, due to the extremely short duration of the contact between strip and zinc, the differences in the height of the zinc column in the stack, while staying within admissible tolerances (of height and, therefore, duration), have the disadvantage that such periodic, noncontinuous top dedrossing procedure introduces discontinuously into the zinc melting oven larger or smaller amounts of top dross and that, as a result thereof, deoxidizing of the top dross also takes place discontinuously and from time to time. It is evident that such discontinuous dedrossing and de-oxidizing has a number of disadvantages and needs improvement.